Sound radiator

ABSTRACT

A series of at least three loud speakers is set one in each of at least three walls forming alternately oppositely opening dihedral angles with each other. The center-to-center distance between the loud speakers is less than 10 times the wavelength of the upper limiting frequency of the sound frequency band. The loud speakers are excited by the same signal source, and a stronger interference field is produced than if the sound emanated from a planar or spherical surface.

United States Patent Huszty et al.

[ Mar. 14, 1972 [54] SOUND RADIATOR [72] Inventors: Denes Huszty; Andras Illenyi, both of Budapest, Hungary [73] Assignee: Elektroakusztikai Gyar, Budapest, Hunga- [22] Filed: Feb. 25, 1970 [21] Appl. No.: 13,886

[30] Foreign Application Priority Data Nov. 26, 1969 Hungary ..EE 1750 [52] US. Cl ..l8l/31 B [51] Int. Cl. ..Gl0k 13/00, I-IO4r 1/28 [58] FieIdotSearch ..181/31R,31A,31B

[56] References Cited UNITED STATES PATENTS 2,544,742 3/1951 Volf..... ....l8l/3l B 2,602,860 7/1952 Doubt.. ....18l/3l B 3,104,729 9/1963 Olson ..18l/3l B 3,179,203 4/1965 Transue ..l8I/3l A 3,241,631 3/1966 Manieri... .....l8l/31 B 3,449,519 6/1969 Mowry ..181/31 B OTHER PUBLICATIONS Publication A Four- Speaker Bass- Reflex Enclosure by Dr. Richard C. Hitchcock, Popular Mechanics, June, I957, pp. l4 2 -l44, l8l 3lB Primary Examiner-Stephen J Tomsky AttorneyYoung & Thompson [57] ABSTRACT A series of at least three loud speakers is set one in each of at least three walls forming alternately oppositely opening dihedral angles with each other. The center-to-center distance between the loud speakers is less than 10 times the wavelength of the upper limiting frequency of the sound frequency band. The loud speakers are excited by the same signal source, and a stronger interference field is produced than if the sound emanated from a planar or spherical surface.

2 Claims, 41 Drawing Figures PAIENTEDHAR 14 I972 3. 648 8 O1 sum UIUF 17 Fig. 7 p

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Fig.3 INVEN'iORS ATTORNEYS PATENTEDMAR 14 I972 3,648,801

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PATENTEDHAR 14 I972 3 648 801 sum GBUF 17 PATENTEDMAR 14 I972 3,648,801

sum 08 or 17 20 50 100 200 500 1k 2k 5k 70k 20k PATENTEDMAR 14 I972 SHEET IOUF 17 PAIENIEDMAR 14 m2 SHEET 110F 17 PATENTEDMARM I972 3,648,801

sum 13 or 17 Fig. 200

PATENTEUHAR 14 I972 3 648 801 sum mar 17 PAIENTEDMAR 14 m2 3, 648.801

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SHEET 17 0F 1? 20 50 700 200 500 7k 2k 5k 70k 20k Fig. 24

SOUND RADIATOR The object of the invention is a sound radiating element and a sound radiator built up of such elements, in which the subjective sensation of the radiated sound is a better one than that of prior sound radiators.

Experts dealing with problems of electroacoustical transmission are well aware of the fact that no high sound power can be obtained from a loudspeaker consisting of only one single sound radiator of small dimensions. By sound radiator it is meant a loudspeaker (s) built in a box, horn, baffle etc. Further it is well known that the radiation properties, i.e., the characteristics of the sound field, the frequency response curve and the directivity pattern (1) (It is to be noted that the parenthetical numerals throughout this text relate to the bibliographical references at the end thereof.) of sound radiators consisting of a number of radiating elements are influenced by the arrangement of the construction. It is to be mentioned that in some cases even one of the sound radiating elements can be used as a sound radiator itself. By such arrangements nonhomogenity sound field, i.e., a sound field with strong interference is energized in the neighborhood of the sound radiator. Its extension, its'nonhomogenity depends on the utilized radiators, their dimensions and their arrangement relative to each other (2). Investigating the difl'erent solutions already realized, a correlation can be observed between the nonhomogenity of the nearfield and the directivity pattern of the radiator. Theoretically the directivity pattern of the surface radiator becomes narrow at high frequencies.

By this the sound pressure is diminished deviating from the axis of the radiator. In FIG. 1, 1 indicates the radiator from side-view, 2 the vector leading to the observers point in the far-field, a() is the angle between the vector of the observers point and that of the axis of the radiator. FIG. 2 shows the idealized frequency response curve p=p(f) in the axis (F and in a certain angle 1! 0 off the axis the directivity pattern gradually diminishes at high frequencies. These theoretical considerations are well proved by practice. It has been the practice to design convex radiators and concave ones too. In FIG. 3, 3 indicates a convex and 4 a concave sound radiator while 2 here also marks the vector of the observers point in the far-field. In such radiators the former effect increases and even the frequency response belonging to the axis (0: =0") is irregular. The corresponding frequency response curves p=p(f of the usual aforementioned arrangements as a function of 11 are shown in FIGS. 4 and 5. In practice in both arrangements further irregularities occur. On convex surfaces between 2 and kHz. a characteristic dip can be observed. On concave surfaces due to the focusing effect in the range of low and middle frequencies where the wavelength, the distance of the focus and that of the measuring microphone are about the same-a maximum of sound pressure occurs. At high frequencies due to the diminishing of the wavelength a diminishing of the focal length can be observed. Hence, the observer or the microphone-as in the case of convex surfaces-is surrounded by a sound field in which the frequency response, as a function of frequency, is falling even in the axis, i.e., at a 0. The fulfillment of the abovementioned considerations may be well observed even in line radiators that can be considered to be a limiting case of surface radiators.

Spherical radiators are to be mentioned particularly as being the most general forms of convex radiators. The classical type of this kind of radiators, the so called 'Kosters radiator (3) was followed by a number of constructions-used in the industry extensivelyamong which the patent descriptions (4), (5) and (6) are to be mentioned as examples. By these radiators a weak interference field is produced in free field, as the elemental radiation has a divergent character. In a living room, i.e., in the neighborhood of reflecting surfaces the interference of the sound field is increased by the reflections from the walls (7), (8), (9). Thus, the subjective sensation of the sound image produced by the sound field is enlarged, the origin of the sound cannot be determined exactly and the effect of the room surrounding the sound source is to be felt considerably.

Although these systems represented a great step forward as compared to prior systems having one single loudspeaker only-which improvement is due to the increased nonhomogenity of the produced sound fieldfor further progress the requirements set in connection with the radiator and the subjective experience have to be revised. In the course of revision efforts are to be directed toward finding which are the desirable characteristics of a sound radiator reproducing sound with remarkable fidelity and the why of it.

In sound radiators the frequency response and the directivity pattern within the transmitted band ought to be smooth and as independent of the frequency as possible, or these conditions should be fulfilled at least within a broad band (10), (l 1). This is an especially important requirement when radiating perspective sound (12). Further to the abovementioned conditions, according to our practice, it is also very important that the frequency responses measured in free field and in a living room should be as equal as possible. In this case namely almost the same sound pressure-and therefore the same subjective sensation-can be obtained in small and in middle size rooms i.e., the reproduced sound image will be independent of the room. This statement can be explained by the equality of the radiated power when the aforementioned conditions are fulfilled.

There emerges, however, the question under what conditions a transmitted band can be considered to be a smooth one.

The program signal to be reproduced-be it speech, music, natural or another sort of noise-in fact, never is a vibration with a line spectrum i.e., a sinusoidal vibration, but always a signal with practically finite bandwidth. The natural sounds, namely, be they speech sounds or musical ones have needs a beginning and an end, i.e., they are of finite duration. Applying Fouriers theorem it is obvious that the ear almost never is excited by pure sinusoidal sounds. Furthennore, in the case of finite duration the spectrum of sounds that could be characterized at infinite duration with a single spectrum line will widen to the bandwidth A f according to the equation A f=2/A t (13). Here A t means the duration of the signal, A f its bandwidth the value of which, in the case of t=50 ms for example, is 40 Hz. The investigations of Winckel (14) show that the phonems of speech and the swift passages of music comprise signals with a duration of about 50 ms. Hence, the system is in fact mostly energized not by a pure sound but by sounds having spectra of considerable bandwidth with a configuration varying with time.

It is well known that the intensities of components within the critical band of hearing (15), at an excitation longer than 10 ms are added by the ear unmindful of the components frequencies within the critical band of hearing (16). The irregularities within the critical band, however, cannot be observed, which fact is proved by the following experience. Let us consider that in a direct hearing, i.e., free of electroacoustical chain, the acoustical surrounding of the listener is an interference sound field, due to reflections on the boundary surfaces and diffraction on objects including the human body (17). The nonhomogenity of this interference field-although considerable-according to practice cannot be observed, not even in an idealized surrounding, free of reflections, in the socalled anechoic chamber. If namely a natural sound source e.g., a speaking person is listened to in an anechoic chamber and the speaking person i.e., the sound source slightly turns away from the listener, the ear of the listener-due to the changing of the geometrical arrangement-is in a different sound field, which fact can be well measured physically, yet the turning of the sound source cannot be observed (18). This phenomenon is obviously due to the fact, that the signals of speechas it is well known-are signals of finite bandwidth. A similar experience can be made when hearing natural sounds, or music, or noise. In a natural surrounding having reflecting surfaces this irregularity will occur even stronger. Let us mention for example the well known highly irregular character of the frequency response curve-measured with the help of sinusoidal signals-of sound radiators operating in a room having reverberation (l9).

Similar conclusions can be drawnamong othersfrom the results of Flohrer (20): according to his experience a dip having a relative bandwidth Af/f 0.1 in the frequency response curve cannot be observed.

It follows from the aforementioned that there is no practical reason for taking into consideration the irregularities of the frequency response within the critical band or the narrow dips having an interference character of the directivity pattern. On the other hand the larger dips and peaks of the frequency response are worth noting, as, when reaching the critical bandwidth, their presenceeven if they are of interference originmay be heard and is of disturbing character (21).

Out of the usual specification for sound radiators let us consider the shape of the frequency response curve within the transmission band. Rather often an irregularity of :5 db. can be observed here. According to Shorters experience (11) irregularities of :2 db. may be observed in some ranges of the frequency response curve.

He observed that for example the quality of the sound-due to the irregularities of a frequency response curve of some db., having dips or peaks broader than the critical bandbecomes colorless, hard and metallic, and that a broad dip at frequencies of 2-5 Hz. provides the sensation of the sound source being far off while at the same place a small size peak gives the impression of the sound source being rather near the observer.

The requirements set in the aforementioned from the subjective side lead to very strict requirements in the technological realization of sound radiators, all the more so, if it is taken into consideration that during monitoring further irregularities are caused in the sound field surrounding the listener.

In the design of the prior sound radiators, on the basis of the aforementioned theoretical considerations, efforts had been directed toward reducing the interference field in order to possibly avoid nonhomogenities. As already mentioned in connection with FIG. 1-5 and when describing the diflerent types of sound radiators, these efforts were only partly crowned by success and even the best loudspeakers have dips and peaks which can be measured objectively, and which can be observed by the listener subjectively. At the same time the task to eliminate the always existing nonhomogenity of the interference field seems to be rather hopeless.

When realizing the object of the invention the basic consideration is not to eliminate the interference field but to create a rather strong one in which dips and peaks occur so frequently that the subjective sensation of their existence is no more possible. Thus, in spite of the fact that these said irregularities of the sound field can be measured objectively, the observer has the subjective impression of being in a homogenous field. Hence, the object of this invention is a sound radiator producing a soundfield the interference nonhomogenity of which is rather strong. If namely the irregularities of the sound field-which is changing in space and time--actually are falling within the critical bands of hearing, these irregularities cannot be observed any more and the subjective sound impression will be a good one. For this purposeon the basis of the aforementioned-such a strong interference field has to be produced at which the condition Aflf 50.1 is satisfied. In this case even the disturbing factors of the room or the surroundings are compensated by the strong inhomogenity of the field, i.e., the electroacoustical transmission becomes almost independent of the surroundings. For better comparison between the subjective sensation and the parameters, measured objectively, it is advantageous to carry out measurements by help of signals having statistical properties like program signals have instead of the common sinusoidal signals. This condition is fairly satisfied by a noise of one third octave bandwidth.

Obviously the abovementioned requirements can only be met with by a compound sound radiating element comprising a plurality of loudspeakers, respectively a sound radiator built up of such elements.

In this invention the strong interference sound field of the element comprising a plurality of loudspeakers (from now: sound radiating element) which is a constituent part of the sound radiator, is produced immediately at the aperture of the sound radiating element. By aperture we means that real or fictive boundary surface of the sound radiating element through which the produced sound field is immediately passed on to the medium outside the sound radiator. In most of the possible arrangements the aperture in the solid angle of the main radiation of the loudspeakers is at one or more points in contact with the wall elements carrying the individual loudspeakers. On the surface of the aperture, being in the nearfield of the sound radiating element, the amplitude and the phase of the excitation of the air particles is rapidly changing from point to point. By this strong nonuniform phase distribution produced along the surface of the aperture as a function of place it is made possible for the sound radiating element to show superdirective properties (22), i.e., to prevent the actual narrowing of the main lobe of the directivity pattern at increasing frequency.

Utilizing this sound radiating element arranged according to the invention as a building block, sound radiators can be composed variable in a wide range with a view toward the actual acoustical task. The sound radiating element may be composed of a plurality or at least three single unit and/or multiple unit loudspeakers arranged in such a manner that the planar surfaces through the radiating surfaces of the loudspeakers have an angle different from and their intersecting lines are parallel, while in the planar surface perpendicular to these intersecting lines the intersecting line is a broken one, so that the radiating (main) axes of the individual neighboring loudspeakers, respectively their projection on the aforementioned perpendicular planar surface are intersecting each other before and behind the broken line alternately, and the radiating centers of the neighboring loudspeakers are at a distance from one another less than three times the wavelength of a sound wave belonging to the upper limiting frequency of the transmitted frequency band.

The upper limiting frequency-according to the draft recommendation of the lntemational Electrotechnical Commission (IEC)is that frequency at which the frequency response of the loudspeaker, measured on the reference axis, has decreased a stated amount (normally 10 db.) below the mean response averaged over a bandwidth of one octave in the region of maximum sensitivity. Sharp peaks and troughs in the response curve narrower than A; octave shall be neglected v for both the upper and the lower limits (23). If over an even number of planar surfaces identical loudspeakers are arranged symmetrically, a radiation with symmetrical directional effect will be obtained, if, however, an odd number of planar surfaces is utilized a special directional effect will result, i.e., the main lobe of the directivity pattern can be tilted. This tilting effect of the sound radiating element utilizing a plurality of loudspeakers is rather weak and is increasing with diminishing the number of loudspeakers building up the sound radiating element and its most expressed form is obtained at an element comprising three loudspeakers.

In fact, the loudspeakers may be arranged asymmetrically too, they can have different dimensions and different forms by which further desirable effects can be attained.

Moreover, to the planar surface over the radiating opening of a loudspeaker-as to aperture planemore than one loudspeaker may be assigned.

The object of the invention shall be described in detail by I way of example with preferred embodiments using the following drawings:

FIG. 6 gives a diagrammatic view of an element arranged according to the invention comprising five loudspeakers and suitable for the explanation of the concept of the invention.

FIG. 7 is the perspective view of an elemental one-line 

1. A sound radiator comprising at least three loudspeakers excited by the same signal source and each having a flat front panel perpendicular to the radiating axis of the speaker, the panels being on a common level and meeting each other at angles substantially different from 180* along lines that are parallel to each other, a flat panel perpendicular to and below and common to all said front panels, said front panels meeting said perpendicular panel in a zigzag line, said axes of adjacent speakers intersecting each other alternately before and behind said zigzag line, adjacent speakers having radiating centers that are at a distance from each other less than 10 times the wave length of the upper limiting frequency of the speakers.
 2. A sound radiator as claimed in claim 1, and a second flat panel perpendicular to and above and common to all said front panels and parallel to the first-mentioned perpendicular panel, said front panels meeting said second perpendicular panel in a zigzag line congruent to the first-mentioned zigzag line. 